There are various different types of engines that use more than one fuel. One type is known as a direct injection gas (DIG) engine, in which a gaseous fuel, such as LPG, is injected into the cylinder at high pressure while combustion in the cylinder from a diesel pilot is already underway. DIG engines operate on the gaseous fuel, and the diesel pilot provides ignition of the gaseous fuel. Another type of engine that uses more than one fuel is typically referred to as a dual-fuel engine, which uses a low-pressure gaseous fuel such as natural gas that is mixed at relatively low pressure with intake air admitted into the engine cylinders. Dual-fuel engines are typically configured to operate with liquid fuel such as diesel or gasoline at full power. The gaseous fuel is provided to displace a quantity of liquid fuel during steady state operation. The air/gaseous fuel mixture that is provided to the cylinder under certain operating conditions is compressed and then ignited using a spark, similar to gasoline engines, or using a compression ignition fuel, such as diesel, which is injected into the air/gaseous fuel mixture present in the cylinder.
When one of the fuel delivery systems in a dual fuel engine malfunctions, for example, when the gaseous fuel supply is unable to provide a sufficient amount of gaseous fuel, the engine adjusts its operation with various degrees of difficulty depending on the type of engine. For example, adjusting engine operation in dual-fuel engines, in which the gaseous fuel supply is at low pressure, is relatively easy because these engines are already configured to operate normally without gaseous fuel. Thus, in the event a malfunction of the engine prevents the supply of the gaseous fuel from reaching the cylinder, the engine merely continues to operate on the liquid fuel.
One example of such an engine configuration can be found in U.S. Pat. No. 6,694,242, which was issued on Feb. 17, 2004. In this example, an engine includes two controllers, one controlling engine operation in a dual fuel mode, and the other to control operation in a single, liquid fuel mode. The engine is configured to operate in diesel only mode under the command of the single-fuel controller. Operation under dual-fuel mode occurs under the control of the second controller. When operating in this mode, gaseous fuel is provided at a low pressure in mixing relation with air entering an engine cylinder, where it is compressed before diesel injected into the cylinder initiates combustion. As can be appreciated, the engine's ability to operate in diesel-only mode allows the engine to transition to this operating mode when, for example, a failure in the gaseous fuel delivery system has been detected.
Another example of dual-fuel engine having a fuel supply that displaces liquid fuel during engine operation can be seen in International Publication No. WO 2007/056845 A1, which published on May 24, 2007. In this example, a gaseous fuel delivery system is designed for retrofit onto existing diesel engines, and includes a gaseous fuel injector installed into an existing glow plug opening of the engine. Gaseous fuel is injected directly into the engine cylinder, as does the existing diesel injector. Combustion is accomplished by preconditioning the air entering the cylinder by use of a heater, which elevates the incoming air temperature to create conditions favorable for gaseous fuel combustion. The engine can operate on both fuels or, with the aid of the heater, in either gaseous fuel-only or diesel fuel-only operation.
Although engines using more than one fuel are known, these engines use the gaseous fuel to displace an amount of liquid fuel such as diesel under certain operating modes. Thus, these engines are capable of operating without any gaseous fuel when a malfunction of the gaseous fuel is detected.
However, this is not the case for DIG engines, in which the main fuel is the gaseous fuel and the secondary liquid fuel, for example, diesel, is used to initiate combustion as previously described. In these engines, a failure of the gaseous fuel system will immobilize the engine and thus the vehicle in which it is installed, because the gaseous fuel is the main fuel supply for the engine and the liquid fuel is merely used as an ignition source. Such immobilization is especially undesired when the engine is installed in large equipment, such as mining trucks, because a single immobilized truck in a fleet can halt an entire mining operation. For example, in mines where roadways to and from a dig site are carved as the dig site moves, a single lane for traffic is typically cut, often at an appreciable grade. A fully loaded truck stranded onto one of these steep roadways will create delays for other trucks entering or leaving the dig site, which will create a hazardous traffic condition and generate long delays and increased costs in the extraction operations of the mine.